The invention pertains to a semiconductor laser with high modulation bandwidth. The invention also pertains to a method for operating a semiconductor laser.
Nowadays, semiconductor lasers are used in various technical fields.
One of these fields is communication engineering. In this case, information such as speech, moving images and still images or computer data is initially converted into electric signals if it is not already present in this form. The electric signals are subsequently used for modulating the brightness of a light source. The thusly obtained light signals can then be transmitted, for example, to a receiver via optical fibers. The receiver converts the received signals into electric signals. If so required, the thusly obtained electric signals are converted further.
If the transmission takes place over longer distances, it may be necessary to install intermediate amplifiers. According to the current state of the art, typical amplifiers of this type initially convert the light signals into electric signals that are then amplified and subsequently emitted anew by a modulated light source.
However, the described method for transmitting data in the form of light signals is not only utilized for transmitting data over longer distances. In the meantime, this type of data transmission is also used in local computer networks (LAN—Local Area Network), last but not least due to the high attainable transmission rates.
In order to achieve the highest data throughput possible on a transmission link, it is certainly advantageous if the light sources capable of modulation used for this purpose have the broadest modulation bandwidth possible. It should also be understood that the term light not only refers to the visible spectrum, but also, in particular, to the infrared range.
Due to their properties, semiconductor lasers are quite suitable for use as light sources capable of modulation in communication engineering. For example, semiconductor lasers have a small structural size, can be manufactured in a relatively cost-efficient fashion and are largely insensitive to environmental influences (particularly to shocks).
However, the modulation bandwidth of semiconductor lasers still poses a problem. For example, conventional semiconductor lasers have a maximum modulation bandwidth on the order of only 10 GHz and therefore the maximum transmission rate on the order of 10 GBit/s.
The aforementioned transmission rates result from the intrinsic high-frequency properties of semiconductor lasers that are essentially defined by the material properties and the layer structure properties, as well as operating parameters such as, for example, the injection current.
One significant limitation of the modulation bandwidth of a semiconductor laser is the recombination time of the charge carriers in the active layer. This recombination time depends on the material properties and the intensity of the interaction between the electron-hole junction and the electromagnetic radiation field in the laser cavity. The corresponding time constant indicates how fast the decoupled light output reacts to changes in the induced charge carrier density, i.e., the modulation current. Since the recombination time of the charge carriers is primarily dependent on the materials in the active layer, the maximum modulation bandwidth can only be conditionally increased.
For example, some improvements with respect to the modulation bandwidth were achieved by respectively providing the laser facets of the semiconductor lasers with an anti-reflective or a highly reflective coating or, alternatively, by realizing the components very short. However, the latter shortens the life of the photons in the cavity. These measures also have the disadvantage that the threshold current density increases significantly, and that the output power is very limited. In order to prevent an excessive drop in the output power, it was attempted to increase the differential amplification (dg/dN, wherein g is the material amplification and N is the charge carrier density), for example, by significantly increasing the active volume with a stack of quantum film layers. The recombination time of the charge carriers can also be reduced, for example, with a p-doping of the active zone.
In any case, the described measures cause a significant increase in the threshold current density and a noticeable deterioration of the laser properties. Until now, the maximum values attained with the described measures in lasers with an emission wavelength on the order of 1.5 μm were approximately 33 GHz.
Due to the unfavorable marginal conditions, directly modulated semiconductor lasers therefore are currently utilized for transmission rates up to approximately 10 GBit/s only.
In order to achieve higher bit rates, it was already attempted to circumvent the problem of the limited intrinsic bandwidth of semiconductor lasers by utilizing external modulators. Bandwidths up to 40 GHz were already attained with this method. However, this technology is very complex (several hybrid optoelectronic components are required) and correspondingly cost-intensive.
Consequently, the invention is based on the objective of proposing a semiconductor laser, in which a high modulation bandwidth is achieved without significantly deteriorating the operating parameters, wherein said semiconductor laser can also be manufactured in a relatively simple and cost-efficient fashion.
This objective is attained with the semiconductor laser and the method for operating semiconductor lasers proposed in the independent claims.
Advantageous additional developments of the semiconductor laser and the method are defined in the respective dependent claims.
In order to obtain a semiconductor laser with high modulation bandwidth, the invention proposes that the semiconductor laser has a substrate that comprises at least three independent functional sections in the direction of light wave propagation. The functional sections are realized, in particular, in the form of an amplification zone (occupation number inversion by means of pumping), a grating zone and a phase adaptation zone. In this context, it should be noted that the term light waves not only refers to light with wavelengths in the visible spectrum, but also, in particular, to infrared waves.
The independent sections respectively fulfill different functions and make it possible to optimally adjust the individual tasks, namely without the optimal adjustment of one task negatively influencing the adaptation of another task. This means that the individual adaptations of the tasks can be respectively solved in an optimal fashion, wherein it is possible, in particular, to adapt the individual tasks in such away that a largely optimal synergetic effect is achieved. A corresponding adjustment of a grating zone or a coupling zone also makes it possible, in particular, to couple the remaining sections to one another in an optimal fashion with respect to the bandwidth of the semiconductor laser such that a largely optimal coupling between the electron-photon resonance (interaction between the electron-hole junction and the electromagnetic radiation field in the laser cavity) and the photon-photon resonance (constructive superposition of the forward and the backward wave in the resonator) can be achieved. Due to this new design concept, the attenuation of the electron-photon resonance can be significantly reduced, and the bandwidth of the semiconductor laser (−3 dB bandwidth) can be shifted to significantly higher values.
Despite the particularly high modulation bandwidth that can be achieved with the proposed semiconductor laser, the inventive laser does not necessarily exhibit the negative effects that were previously associated with an increase in the modulation bandwidth, for example, an increase in the threshold current density and a reduction of the output power.
In the aforementioned design concept, it would also be conceivable for one functional section to fulfill two functions. For example, the grating zone that serves for the wavelength selection may simultaneously serve, in particular, as a coupling element between the amplification zone and the phase adaptation zone.
A particularly simple manufacturing process is achieved if the substrate of the semiconductor laser is realized in the form of a monolith. In addition, undesirable reflections or phase shifts usually do not occur at the junctions between different substrate regions. Corresponding impedance adaptation junctions may have to be provided, if so required.
It is advantageous that the semiconductor laser is provided with at least one waveguide region. This makes it possible to achieve a defined propagation of the light wave in the semiconductor substrate. If so required, different regions of the waveguide may have different wavelength dispersions (also with respect to the sign) such that wave packets exhibit the least dispersion possible over the total length of the semiconductor laser.
If the individual functional sections essentially fulfill only one function, the adaptation of these functions can be additionally improved.
An advantageous coupling between the amplification zone and the phase adaptation zone can be promoted if a grating zone, a coupling zone or both (if applicable, in the form of a combined grating and coupling zone) is arranged between the amplification zone and the phase adaptation zone. This usually results in a particularly high modulation bandwidth.
It is particularly advantageous if only a weak coupling between the light wave A and the grating structure results in the grating zone. The invention proposes, in particular, a coupling coefficient κ of κ≦60 cm−1, preferably between κ=10 cm−1 and κ=50 cm−1, particularly between κ=20 cm−1 and κ=40 cm−1. Since the coupling is comparatively weak, the transmittance of the light wave through the grating zone is ensured. This is particularly important if the grating zone is arranged between the amplification zone and the phase adaptation zone as described above. In addition, the weak coupling makes it possible to easily choose the filter curve of the grating zone in a sufficiently, but not excessively selective fashion by providing a corresponding number of grating structure elements.
In case of an active grating zone, an active coupling zone or both, i.e., if the coupling between the light wave and the grating structure as well as the attenuation and the coupling properties (coupling through the section in question) of the respective zones can be varied, it is possible to additionally promote a mutually optimal adaptation of the individual functional sections. When carrying out an adaptation of an active zone, it may occur that the change of one property (e.g., the coupling between the light wave and the grating structure) results in a change of another property (e.g., the attenuation of the light wave while it passes through the grating).
It also proved advantageous to design the grating structures in such a way that they are realized laterally referred to the waveguide region. This measure makes it possible, in particular, to achieve the described weak coupling between the light wave and the grating in a particularly simple fashion.
It is also advantageous that the semiconductor laser has a component length that corresponds to that of conventional semiconductor lasers, namely a component length of more than 0.5 mm, preferably more than 0.8 mm, particularly more than 1.0 mm. This measure makes it possible to eliminate the need for an excessively high differential current amplification that could lead to correspondingly disadvantageous operating parameters. Excessive injection current intensities and a limited laser output power, in particular, can be prevented in this fashion.
The invention also proposes a corresponding method for operating a semiconductor laser that makes it possible to achieve a particularly high modulation bandwidth. In this context, the invention proposes that different tasks such as, in particular, amplification (occupation number inversion by means of pumping), phase adaptation, wavelength selection and/or attenuation are carried out in at least three independent sections along the substrate material of the semiconductor laser. This method provides the same advantages as those described above with reference to the proposed semiconductor laser.
The proposed method can be additionally developed in a particularly advantageous fashion if the individual sections are adapted such that a largely optimal coupling between the electron-photon resonance and the photon-photon resonance of the semiconductor laser is achieved. This additionally promotes a particularly high modulation bandwidth of the semiconductor laser.
It is also advantageous that the semiconductor laser is operated on the side of longer wavelength of the reflection peak. This measure makes it possible, in particular, to easily prevent a chirp (i.e., a frequency modulation of the semiconductor laser that results from a changed amplitude), namely because the modulation of the optical index of refraction is normally at its lowest in this case. Since a frequency chirp results in a temporal superposition of the pulses after a certain transmission distance due to the dispersion of a following optical fiber, an additionally improved transmission rate can be realized with the proposed additional development.